SDK / exoplayer

Simplify the audio encoder input timestamp calculation. The new calculation
avoids drifting by tracking the total number of bytes encoded rather than
tracking the timestamp and remainder separately, and also makes the timestamps
match the decoder output buffer timestamps.

Also switch one of the export tests that was passing through AMR samples over
to using WAVE audio. The problem with using AMR is that the compressed samples
are not necessarily an integer number of audio frames and the shadow decoder
would pass them from input to output, so the audio encoder was receiving
non-integer numbers of audio frames.

Tested by logging the timestamps at the decoder output and encoder input with
forcing transcoding audio, and verifying that after this change the audio
timestamps are no longer off by one.

PiperOrigin-RevId: 523409869